In recent years, in internal combustion engines mounted to vehicles, air-fuel ratios of respective cylinders (cylinder-by-cylinder air-fuel ratios) are calculated on the basis of an output of an air-fuel ratio sensor provided to an exhaust manifold, through which exhaust gases from the respective cylinders flow, as described in, for example, JP-B2-2684011. Here, air-fuel ratios are subjected to feedback control for each cylinder on the basis of air-fuel ratios of respective cylinders. A cylinder or cylinders being abnormal in air-fuel ratio are detected on the basis of air-fuel ratio feedback correction terms of the respective cylinders.
Besides, in recent years, internal combustion engines mounts variable valve lift devices, which vary lift of intake valves and exhaust valves, and variable valve timing devices, which control opening and closing timings. The variable valve lift devices and the variable valve timing devices are for the purpose of an enhancement in intake efficiency, an enhancement in output, reduction in fuel consumption, reduction in exhaust emission, or the like in internal combustion engines mounted to vehicles. Variable valve lift devices are various in construction such that valve lifts of multiple cylinders are varied together by a single hydraulically driven variable valve lift device. Alternatively, electromagnetically driven valves are provided to each cylinder to independently vary valve lift for each cylinder. An abnormality diagnosis system for detection of abnormality is also demanded of such respective devices, similarly to other air-fuel ratio control systems.
In the abnormality diagnosis system disclosed in JP-B2-2684011, a cylinder or cylinders being abnormal in air-fuel ratio are detected on the basis of air-fuel ratio feedback correction terms of respective cylinders. Here, the cause for abnormality in air-fuel ratio of the cylinder or cylinders is considered to be based on abnormality in a fuel system such as fuel injection valves, abnormality in an air system, abnormality in an ignition system, or the like. Therefore, even when a cylinder or cylinders being abnormal in air-fuel ratio can be detected, the cause for abnormality in air-fuel ratio of the cylinder or cylinders is unclear. Accordingly, in the abnormality diagnosis system disclosed in JP-B2-2684011, even when variable valve lift devices fail to cause abnormality in air-fuel ratio of the cylinder or cylinders, the cause of the abnormity cannot be determined to perform an appropriate fail-safe processing.
Besides, with hydraulically driven variable valve lift devices, valve lifts of multiple cylinders are varied by a single variable valve lift device. In case of an increase in number of cylinders, it is difficult to ensure a sufficient hydraulic drive pressure with a single variable valve lift device. Therefore, cylinders are divided into multiple cylinder groups, so that variable valve lift devices, which are hydraulically driven independently, are provided to each cylinder group. In a system having multiple variable valve lift devices, only a variable valve lift device of one cylinder group may become abnormal. Accordingly, an abnormality diagnosis system is demanded, such that the abnormality diagnosis system can determine which variable valve lift device of cylinder groups is abnormal.
When the abnormality diagnosis system disclosed in JP-B2-2684011 is applied to such system having multiple variable valve lift devices, the following problems may arise.
In the system, air flowing through a single intake pipe is distributed to respective cylinders through intake manifolds of the respective cylinders. Therefore, when variable valve lift devices in one of cylinder groups fail, and an intake air quantity of the fault cylinder group is disturbed, intake air quantities of the other normal cylinder groups are disturbed. As a result, not only an abnormal cylinder group but also normal cylinder groups are disturbed in air-fuel ratio, so that a cylinder or cylinders being abnormal in air-fuel ratio may be erroneously detected.
Hereupon, sensors may be provided to variable valve lift devices of respective cylinder groups for respectively detecting lift and hydraulic pressure, so that it may be determined that which variable valve lift device of the cylinder groups is abnormal on the basis of outputs from the sensors. However, sensors for detection of lift and hydraulic pressure need to be newly provided, and cost may be increased.
In abnormality diagnosis of variable valve timing devices described in JP-A-9-88643, an intake air quantity Q1 is detected just before the start of abnormality diagnosis, and is stored in memory, subsequently, abnormality diagnosis is started. In abnormality diagnosis, variable valve timing devices are forcedly driven, and an intake air quantity Q2 is detected for calculation of a variation (Q1–Q2) of an intake air quantity before and after the start of abnormality diagnosis. By comparing the calculated value with an abnormality threshold, normality/abnormality of the variable valve timing devices is determined.
However, in the abnormality diagnosis system disclosed in JP-A-9-88643, abnormality diagnosis is not executed as long as a predetermined abnormality diagnosis execution condition is not met in an engine operation. Accordingly, even when abnormality is caused in variable valve timing devices, the abnormality may not be early detected.
As a countermeasure, it is conceivable to relax the abnormality diagnosis execution condition to increase a frequency, in which abnormality diagnosis is executed in engine operation. In a method of abnormality diagnosis disclosed in JP-A-9-88643, variable valve timing devices are forcedly driven. However, when the frequency, in which abnormality diagnosis is executed, is increased, harmful influences, such as torque shock caused by forced driving of variable valve timing devices are increased. As a result, operability becomes worse.
Hereupon, instead of forced driving of variable valve timing devices, it is conceivable to monitor the behavior of an engine rotating speed, an intake air quantity, and the like in engine operation to early detect an abnormal behavior. However, with engines having multiple intake efficiency varying means, which vary the intake efficiency, such as variable valve timing devices, variable valve lift devices, and the like, even when an abnormal behavior in intake air quantity and the like is detected in engine operation, it cannot be determined that which of multiple intake efficiency varying means is abnormal.